1. Field of the Invention
The present invention relates to a group III-nitride semiconductor thin film, a method for fabricating the same, and a group III-nitride semiconductor light emitting device, and more particularly, to a thin film which can serve as a base layer for the epitaxial growth of GaN-based compounds thereon.
2. Description of the Related Art
Group III-nitride semiconductors, more particularly, GaN-based compounds, have an ability of widely controlling an energy gap by regulation of a composition ratio thereof. As an example, GaN-based compounds, which have the formula AlxInyGa(1-x-y)N (where 0≦x≦1, 0≦y≦1, x=y=0), act as a direct transition semiconductor, and have an energy gap in a range from 0.7-0.8 eV to 6.2 eV. Accordingly, it can be understood that utilizing GaN-based compounds in the formation of an active layer makes it possible to realize a light emitting device capable of emitting light of all colors in a visible range from red-color light to ultraviolet light.
To apply GaN-based compounds to a light emitting device, there is a necessity of a thin film having a high quality and high luminous efficiency in the viewpoint of the shape or lifespan of products. As known, GaN-based compounds have a hexagonal-wurtzite structure and the low lattice constant of GaN-based compounds shows a great difference with other main semiconductors (group III-V compound semiconductor, group II-VI semiconductor, etc.). The very low lattice constant is difficult to match to the lattice constant of a substrate crystal. Generally, if the lattice constant of a substrate crystal differs from the lattice constant of a crystal to be epitaxially grown on the substrate crystal, the resulting growth layer is easily affected by a compressive bending or tensile bending and tends to disadvantageously accumulate an elastic bending energy therein. Although the elastic energy is within an allowable range if the growth layer is thin, it may generate an electric potential if the thickness of the growth layer exceeds a certain value, thus causing lattice relaxation and resulting in a great amount of electric potential defects in the growth layer. For this reason, selection of a substrate is important in the growth of GaN-based compounds.
Conventionally, a c-plane sapphire substrate has been used as a substrate for growing GaN-based compounds thereon because it has a lattice constant most close to that of GaN. However, the lattice constant of the c-plane sapphire substrate still has a difference up to approximately 15% with that of GaN. Therefore, in order to alleviate the effect of lattice mismatch, a buffer layer has been actually formed between the sapphire substrate and a growth layer. Nowadays, the quality of the buffer layer becomes an important factor of determining the quality of a growth layer on the buffer layer. A variety of buffer layers have been proposed as a result of continuous research and development (Ref. Japanese Patent Publication Nos. Heisei 10-242586 and 9-227298).